Women’s brains could be more than six years younger than men’s which may explain why they are more likely to stay mentally sharp as they age, a new study found.

As we age our brains shrink and men’s diminish faster than women’s.

The brain’s metabolism also slows as people grow older, and this, too, may differ between men and women.

Researchers found that women’s brains were on average 3.8 years younger than their age, whereas men’s were 2.4 years older.

This means a woman could have a brain 6.2 years younger than a man of the same age.

The study involving brain scans by Washington University School of Medicine in St. Louis found women’s brains appear to be about three years younger than men’s of the same chronological age, metabolically speaking.

Senior author Assistant Professor Dr Manu Goyal said: “We’re just starting to understand how various sex-related factors might affect the trajectory of brain ageing and how that might influence the vulnerability of the brain to neurodegenerative diseases

“Brain metabolism might help us understand some of the differences we see between men and women as they age.”

Prof Goyal explained brain runs on sugar, but how the brain uses sugar changes as people grow and age.

Babies and children use some of their brain fuel in a process called aerobic glycolysis that sustains brain development and maturation.

The rest of the sugar is burned to power the day-to-day tasks of thinking and doing.

In adolescents and young adults, a considerable portion of brain sugar also is devoted to aerobic glycolysis, but the fraction drops steadily with age, leveling off at very low amounts by the time people are in their 60s.

To understand the differences in brain metabolism between the sexes 121 women and 84 men aged between 20 to 82 were enrolled to see how their brains use sugar.

Brain scans measured the flow of oxygen and glucose in their brains and for each person, the researchers determined the fraction of sugar committed to aerobic glycolysis in various regions of the brain.

A machine-learning algorithm then found a relationship between age and brain metabolism by feeding it the men’s ages and brain metabolism data.

Then, the researchers entered women’s brain metabolism data into the algorithm and directed the programme to calculate each woman’s brain age from its metabolism.

The algorithm yielded brain ages an average of 3.8 years younger than the women’s chronological ages.

The researchers also performed the analysis in reverse training the algorithm on women’s data and applied it to men’s.

This time, the algorithm reported that men’s brains were 2.4 years older than their true ages.

Prof Goyal said: “The average difference in calculated brain age between men and women is significant and reproducible, but it is only a fraction of the difference between any two individuals

“It is stronger than many sex differences that have been reported, but it’s nowhere near as big a difference as some sex differences, such as height.”

The relative youthfulness of women’s brains was detectable even among the youngest participants, who were in their 20s.

Prof Goyal said: “It’s not that men’s brains age faster - they start adulthood about three years older than women, and that persists throughout life.

“What we don’t know is what it means.

“I think this could mean that the reason women don’t experience as much cognitive decline in later years is because their brains are effectively younger, and we’re currently working on a study to confirm that.”

Older women tend to score better than men of the same age on tests of reason, memory and problem solving.

Future studies are planned to follow a cohort of adults over time to see whether people with younger-looking brains are less likely to develop cognitive problems.

The study was published in the journal Proceedings of the National Academy of Sciences.